This invention relates to a method of carding a fibrous feedstock to form a carded sliver using a carding engine having a main toothed wire cylinder, a takerin, a doffer, and a series of flats movable along a closed loop having a working path arranged alongside the outer periphery of the cylinder between the takerin and the doffer in order to carry out a carding operation on the feedstock in co-operation with the teeth of the rotating cylinder and to remove trash particles, nep, dust and short fibres from the feedstock to form the carded sliver.
In commercial production of carded webs to form a sliver, it is usual to run the cylinder such that the circumferential speed of the cylinder periphery is about 60,000 inches per minute. Thus, for a 40 inch diameter cylinder, a typical speed of revolution would be about 500 rpm. These figures are given by way of general explanation only, and typical cylinder dimensions and operating speeds vary from one manufacturer to another.
By contrast, a typical linear speed of the revolving flats along the working path is of a very considerably lower order, with a typical speed being about 6 inches per minute i.e. the ratio of flats speed X to cylinder peripheral speed Y is about 1:10,000.
As is well known in the carding art, the fibrous feedstock fed to the cylinder by the takerin is conveyed into the carding zone where the combined action of the high speed cylinder wires or teeth and the much slower speed wires or teeth of the flats exert a carding action on the feedstock to form a carded web, while in addition the flats remove unwanted material from the feedstock in the form of trash particles, nep, dust and short fibres which constitute flat strip waste.
The waste will include small fragments of seeds, husks and also undesired short fibres, so that the resulting web is composed mainly of fibres of usable length. The required amount of short fibres to be removed will depend upon the required quality of sliver to be derived from the web, and the operating parameters will be set-up to suit particular sliver quality requirements.
As mentioned above, the speed of the flats is very small compared to the peripheral speed of the cylinder, and it has always been assumed that this order of flats speed was. necessary in order to carry out a carding operation which is both technically efficient in producing a carded web which can produce a sliver of required quality, and also economic in its usage of the raw material feedstock.
With regard to the matter of economic usage of the raw material, the quality of the sliver depends upon the proportion of short fibres initially present in the feedstock, which still remain in the carded web after carding treatment, and therefore it is desirable to operate the carding engine in such a way as to minimise the residual short fibre content in the web. However, it has always been conventional wisdom that any alteration to the operating parameters to achieve increased removal of short fibres will necessarily involve an unacceptable raw material cost penalty by reason of an inevitable increase in extraction of fibres of usable length (typically about 1 inch).
Therefore, typical running speeds of flats of about 6 inches per minute are considered to be an acceptable compromise between the conflicting requirements of (a) maximising the cleaning efficiency and removal of short fibres and other undesired waste from the feedstock to obtain a required quality of sliver and (b) minimising the loss of acceptable fibres from the feedstock to avoid commercial under utilisation of the raw material resource.
Such typical running speeds have been used for many years, and have been found to provide a satisfactory quality of sliver consistent with economic utilisation of the raw material, and further, and most importantly, have provided reliable operation over extended periods, during which the radial clearances between the flats and the cylinder periphery remain within acceptable limits. The maintenance of these clearances is very important to consistent sliver quality, and it is desirable for the flats and cylinder to run for time intervals as long as possible before it becomes necessary to carry out adjustment or re-grinding to re-establish the clearances within the acceptable limits.
One of the major causes of clearance variations over a period of time is wear between the flat ends and "flexible bends" which control the path of the flats around a closed loop, and a low running speed of the flats for a given speed of cylinder contributes to low wear. Therefore, for all of these reasons, and maybe others, it is totally accepted practice that flats run at speeds of the order indicated.
Despite the accepted wisdom prevailing amongst carding machine manufacturers and operators concerning the running speed of flats, the applicants did make some tentative experiments a number of years ago into possible variation in sliver quality which would result by reason of increasing the linear speed of the flats while maintaining normal production speeds of the cylinder. The results were considered at the time to be unsatisfactory, as there was unacceptable loss of long fibres which were carried over into the waste and which was discovered on inspection of the waste strips. Furthermore, there was no obvious improvement in the quality of the sliver, and no further work was done on this project. The experiment thus simply confirmed the prejudice against any contemplation of the assumed practicality of increasing the flats speed.
However, the applicants have recently undertaken further research on high speed flats and, despite repeated negative indications of improvement in the fibre profile of the sliver, nevertheless this has been pursued to the stage of knitting cloth using a rotor yarn spun from sliver produced by a carding engine on which the flats were run at high speed. The very surprising result of this has been to find that the quality of knitted fabric from a yarn derived from a sliver obtained on a single carding engine which is operated at increased speed of the flats is considerably improved as compared with the same carding engine operating at normal speeds. On detailed inspection of such fabric quality parameters as "evenness" and "hairiness" etc, the quality was found to be comparable to the quality of fabric obtained from so-called "scratch combed" quality yarn, i.e. yarn derived from a sliver which has been subjected to three separate process stages, namely carding in a conventionally operated carding engine followed by subsequent combing.
The reason why this highly surprising advantageous result is obtained is not clear at present, but experimental data derived to date clearly indicates that this result is achieved when the speed of the flats is increased substantially.
One possible explanation is that the key to the improvement lies in the making of a substantial increase in the ratio of flat speed X to cylinder peripheral speed Y, the increase being from a typical current value of 1:10,000 to a higher value i.e. at least 1.5:10,000, more preferably in the range of up to ten times greater, namely 1:1,000 and possibly even higher.
A further possible explanation is that the key factor is solely the increase in speed of the flats as such, regardless of cylinder speed. Thus, significantly advantageous results may be obtained with an increase of flat speeds to 16 inches per minute or more.
A still further possible explanation, which may be related to either or both of the above, is that the fibre tuft material caught by the high speed flat, and exposed to the wire action of the cylinder, is subjected to a lower number of cylinder wires acting on it than would be the case during the longer exposure time at a lower flats speed.